The present invention relates generally to inductors and, more particularly, to a high performance induction melting coil encapsulated with blocks or inserts fabricated from a low reluctance composition useful in controlling the direction of the inductor's flux density.
Inductors or inductor coils are generally used to heat a workpiece made of a conductive material via currents induced by varying an electromagnetic field. As such, electromagnetic energy is transferred from the inductor to the workpiece. More particularly, as alternating current from a power source flows through the inductor coil, a highly concentrated magnetic field is established within the coil. The strength of the magnetic field depends primarily on the magnitude of the current flowing in the coil. Thus, the magnetic field induces an electric potential in the workpiece and, since the workpiece represents a closed circuit, the induced voltage causes the flow of current. These induced currents are commonly called eddy currents such that the current flowing in the workpiece can be considered as the summation of all of the eddy currents. Resistance of the workpiece to the flow of the induced current generates heating by I.sup.2 R losses. Therefore, heat is generated in the workpiece by hysteresis and the eddy current losses, with the heat generated being a result of the energy expended in overcoming the electrical resistance of the workpiece. Typically, close spacing is used between the inductor coil and the workpiece, and high coil currents are used to obtain maximum induced eddy currents and resulting high heating rates.
Induction heating is widely employed in the metal working industry to heat metals for soldering, brazing, annealing, hardening, forging, induction melting and sintering, as well as for other various induction heating applications. As compared to other conventional processes, induction heating has several inherent advantages. First, heating is induced directly into the conductive workpiece for providing an extremely rapid method of heating. Furthermore, induction heating is not limited by the relatively slow rate of heat diffusion associated with conventional processes using surface contact or radiant heating methods. Second, because of a "skin" effect, heating is localized and the area of the workpiece to be heated is determined by the shape and size of the inductor coil. Third, induction heating is easily controllable, resulting in uniform high quality heat treatment of the product. Fourth, induction heating lends itself to automation, in-line processing, and automatic process cycle control. Fifth, start-up time is short, and thus standby losses are low or nonexistent. And sixth, working conditions are better because of the absence of noise, fumes, and radiated heat. As will be appreciated, numerous other advantages exist for selecting induction heating over conventional heating processes.
Modernly, induction melting has gained wide-spread acceptance in the metal working industry as the method of choice due to its previously-noted advantages. Traditionally, "coreless" melting coils have been fabricated by winding copper tubing around a mandrel having a predetermined shape. Thereafter, a plurality of studs are brazed to an outer peripheral surface of the copper tubing. The studs are secured by suitable fasteners to phenolic or wooden stud board for rigidly maintaining the turns of the melting coil in a predefined spacial relationship. As is known, an inherent tendency exists for the lines of magnetic flux generated by the melting coil to inductively couple with any surrounding conductive materials (such as when the melting coil is placed within a vacuum chamber) which, in turn, heats the surrounding conductive material and/or interferes with operation of surrounding control systems. It is also well known that the magnetic flux generated by the inductor must be dense enough to bring the workpiece to a desired temperature in a specified time (typically short).
In the past, it has been recognized that the performance of induction melting coils may be improved by controlling the direction of flux flow and thereby manipulating and maximizing flux density on the workpiece. Conventionally, with an induction melting coil of generally circular cross-section, directional control was thought to be improved by attaching laminated stacks of flux controlling elements or "shunts" on certain portions of the circumference, so that the magnetic flux is intensified on the corresponding area of the workpiece. Typically, such shunts include laminations made of grain-oriented iron (which are generally made from relatively thin pieces of silicon steel strip stock) and which are attached to the inductor on a strip by strip or layer by layer basis as necessary. While generally satisfactory for shielding or "blocking" the field from heating surrounding conductive components, shunts are generally unsatisfactory to the extent that they are difficult to apply, requiring cutting and sizing to the necessary configuration. Thus some portions or parts of an inductor cannot be covered because of the difficulty of application. Applying "shunt" laminations to large inductors is also somewhat prohibitive due primarily to excessive cost and labor considerations. In addition, these iron laminations have a tendency to lose permeability at high operating temperatures which results in inefficient heating operations. Furthermore, at higher temperatures, the shunts require cooling due to relatively high hysteresis and eddy current losses.
Accordingly, the present invention relates to improved inductors and, more specifically, to improved induction melting coils encapsulated with block or inserts fabricated from a composition useful in controlling the direction of inductor flux density. In this manner, the induction melting coil is encapsulated to provide a low reluctance path within which the magnetic field travels while "blocking" inductive coupling of the magnetic field with surrounding auxiliary components.
In a preferred form, the flux concentrator blocks of the present invention are made of a composition employing a high purity, annealed, electrolytically prepared iron powder with a unique physical characteristic and a polymer binder which includes a resin or mixture of resins. The compositions may optionally employ an additional material or component such as an acid phosphate insulating coating. The flux concentrator inserts or blocks fashioned from the resulting compositions provide improved performance when employed in induction melting modalities over conventional, art-disclosed shunt materials in that the inserts formed from these compositions maintain the necessary permeability and demonstrates a maximum of about sixty (60) percent regression in permeability between the commonly employed frequencies of 10 KHz and 500 KHz and a total core loss of less than about 0.8 to about 1.2 ohms in this range. The iron powder in the compositions of the present invention is characterized in that it is substantially non-spherical and generally flat or disc-shaped and possesses a specific surface area of less than about 0.25 m.sup.2 /g. The iron powder described above is particularly well-suited for use in induction melting coils in that it permits the formation of a relatively thick and rigid flux controlling insert by pressing at relatively high pressures such that the insert possesses a very high density with an extremely high ratio of ferromagnetic material to binder material while still permitting the binder material to perform well.
Further objects and advantages of the present invention will become apparent to one skilled in the art from examination of the following written description taken in conjunction with the accompanying drawings and the appended claims.